This invention relates to micro-electro-mechanical systems (MEMS), and more particularly to MEMS scanning mirrors.
Various electrostatic designs for MEMS scanning mirrors have been proposed. Their applications include barcode readers, laser printers, confocal microscopes, fiber-optical network components, projection displays for projectors, rear projection TVs, wearable displays, and military laser tracking and guidance systems. Typically a MEMS scanning mirror is driven at its. main resonance to achieve a high scan angle. Invariably the manufacturing processes produce MEMS scanning mirrors with dimensional inconsistencies that vary the natural frequencies of the individual devices. If the main natural frequency of a minority of the MEMS scanning mirrors is too low or too high, the minority devices will not produce the proper scan speed and the proper scan angle under an alternating current (AC) drive voltage selected for a majority of the MEMS scanning mirrors. Thus, an apparatus and a method are needed to tune the main natural frequency of the MEMS scanning mirrors to improve the manufacturing yield of these devices.
In one embodiment of the invention, a MEMS structure includes a first electrode, a second electrode, and a mobile element. The first electrode is coupled to a first voltage source. The second electrode is coupled to a second voltage source. The mobile element includes a third electrode coupled to a third voltage source (e.g., an electrical ground). A steady voltage difference between the first electrode and the third electrode is used to tune the natural frequency of the structure to a scanning frequency of an application. An oscillating voltage difference between the second electrode and the third electrode at the scanning frequency of the application is used to oscillate the mobile element. In one embodiment, the mobile element is a mirror.